There's two big issues with most reproductions of the Vesper you see today. One is that virtually all use 80 proof vodka and gin because that's the default - Fleming's version would have been 100 proof. The other, more important, issue is that Kina Lillit hasn't been made since 1986, and the Lillit Blanc usually used as a substitute has a dramatically different flavor profile. Experiments trying to factor that in have produced tolerable results by some reports.

That said, the primary reason the Vesper in that form (as opposed to a simple Vodka Martini) never appears again is the Fleming copied thenrecipe from a friend and hadn't tried it before including it in the novel. When he did, he disliked it immensely.